The present invention relates generally to radar receivers, and more particularly to a method and apparatus for distinguishing between signals received from a desired target and those received from unwanted clutter.
It is well known that radar operate by transmitting a signal at a radio frequency (hereinafter referred to as the transmitted signal). The transmitted signal is reflected by a target and returned as a received signal. A received signal returned by a target moving radially with respect to the radar transmitter has a frequency different from that of the transmitted signal. The difference between the frequency of the transmitted signal and the received signal, known as the Doppler shift frequency, may be used to determine the range rate between the target and the transmitter. If several targets having different range rates are illuminated by the transmitted signal, the resulting received signals returned by each target are distributed within a band of Doppler shift frequencies. The location of the received signal returned by each target within the Doppler frequency band is thus a function of the range rate of the associated target. If the range rate of a particular desired target is greater (or less) than the range rate of certain other undesired targets, filtering techniques may be used to separate the received signal returned from the particular desired target from the received signals returned from undesired targets.
A problem occurs when the Doppler frequency of a desired target is close to the undesired target""s Doppler frequency, either directly or as observed ambiguously with a given waveform. This is particularly troublesome when the undesired targets include ground clutter, as received signals returned from ground clutter are often much more powerful than received signals returned from desired targets. This invention addresses the problem of either acquiring or tracking a low altitude target using a Medium Pulse Repetition Frequency (MPRF) radar on a moving platform, such as an aircraft or missile. The problem faced by such a radar is that when the radar is looking down on the target, mainlobe clutter will always appear in the range, Doppler output of the radar. The mainbeam of the radar""s antenna can cover a significant range swath, leaving only the Doppler dimension to resolve the target from mainlobe clutter. If the target is Doppler ambiguous with the Doppler of mainlobe clutter, it will be masked by the clutter, and the radar will be unable to derive useful information from the target signal. Through judicious choice of waveform PRF, the mainlobe clutter blind can be avoided. During track, the radar can resolve the true Doppler of the target and mainlobe clutter, and select PRFs that avoid the clutter blind. During search, multiple PRFs are generally required to uncover the mainlobe clutter blind in each search beam position unless the target Doppler has been accurately designated to the radar.
With the foregoing background of the invention in mind, it is a general object of the present invention to improve the detectability of small desired targets in the presence of powerful ground clutter in radar systems.
In accordance with the present invention, a method for operating a radar system includes the steps of overlapping mainlobe clutter interference and feedthrough interference in a Doppler output signal of a coherent pulse Doppler radar including the step of phase modulating each pulse of a transmit signal wherein the selected phase modulation is derived from a linear phase ramp required to shift the mainlobe clutter interference into alignment with the feedthrough interference while being held constant over the duration of a pulse. With such an arrangement, the constant phase shift across each pulse has no effect on the performance of intrapulse compression modulation and results in pulse to pulse phase shifts easily being accomplished with existing phase shifters. The technique takes advantage of the phase run out over the duration of a pulse between the applied modulation and linear phase ramp produced by the Doppler shift of the mainlobe clutter signal. The phase run out is a phase ramp with periodicity exactly matching that of the transmitted waveform with the resultant Phase Modulation (PM) line spectrum aliasing at the waveform PRF thus causing mainlobe clutter signal to alias into the feedthrough signal.